Elastomeric composition using monomeric distillate by-product

ABSTRACT

The specification discloses a method for vulcanizing an elastomer using as the cure promoter the monomeric distillate by-product from the clay-based dimerization of an unsaturated fatty acid mixture, preferably a tall oil fatty acid mixture. The distillate provides a cost-effective alternative to conventional fatty acid promoters.

This application is a division of application Ser. No. 08/801,758, filedFeb. 18, 1997 now U.S. Pat. No. 6,072,009 which is acontinuation-in-part of application Ser. No. 08/463,292, filed Jun. 5,1995, now U.S. Pat. No. 5,756,619, which is a division of applicationSer. No. 08/233,351, filed Apr. 26, 1994, now abandoned.

The present invention relates to cure promoters and methods forvulcanizing elastomers.

Many uses of elastomeric materials require that the polymer chains inthe elastomer be cross-linked. Crosslinking of the polymer chains in anelastomeric matrix is typically referred to as curing or vulcanization.Crosslinking is commonly accomplished using sulfur or an organic sulfurcompound at an elevated temperature. As a result of crosslinking, thecured elastomer is elastic, i.e., it deforms under stress yet returns tothe shape it had when the stress is removed, has improved tensilestrength, reduced temperature sensitivity, higher elongation, greatertear resistance and increase hardness or durometer.

During the vulcanization reaction, elastomers are typically reacted withsulfur in the presence of a cure activator or promoter. Commonly usedcure activators are oleic and stearic acids combined with zinc oxide anda cure accelerator. However, oleic and stearic acids are relativelyexpensive and since they are used in large quantities in rubberformulations, they contribute to the high cost of many elastomericproducts.

Various alternatives to stearic acid have been proposed including theuse of tall oil fatty acid mixtures as described in U.S. Pat. Nos.4,870,135 and 4,895,911 to Mowdood et al. However, these products arealso relatively expensive and are subject to price fluctuations whichlead to market instabilities.

It is therefore an object of the invention to provide a cost effectivecure promoter and method of using the same for vulcanizing elastomericmaterials.

Another object of the invention is to provide a low cost alternative tostearic acid as a cure promoter for an elastomeric material.

A further object of the present invention to provide a cure promoter foran elastomeric formulation which is readily available and which may besubstituted directly for stearic acid in an elastomeric compositionwithout adversely affecting the properties of the resulting curedelastomer.

An additional object of the invention is to provide a method forpromoting the cure of elastomers using a relatively inexpensive curepromoter.

Having regard to the above and other objects, the invention provides amethod for making a cross-linked elastomer which comprises compounding amixture including a vulcanizable elastomer, a sulfur source and fromabout 0.1 to about 6% by weight of a monomeric distillate by-productfrom the clay-catalyzed dimerization of fatty acids. The compoundedmixture is then heated to a temperature sufficient to cause substantialcrosslinking of the elastomer.

The monomeric distillate by-product used in the present invention is alow cost alternative to stearic acid, oleic acid and tall oil fatty acid(TOFA) and is available in plentiful supply. The monomeric distillateby-product may be obtained from the clay-catalyzed dimerization of fattyacids obtained from oils selected from the group consisting of linseed,canola, soybean, rapeseed and tall oils or blends of the fatty acidsobtained from two or more of the foregoing oils. It is especiallypreferred to use the monomeric distillate by-product from theclay-catalyzed dimerization reaction of tall oil fatty acids (TOFA).Although TOFA is produced in large quantities from renewable resourcesand is itself a known cure promoter for vulcanization, the monomericdistillate by-product from the clay-based dimerization of TOFA has notbeen recognized as having utility in such applications. The compositionof the distillate is quite different from that of either the dimerproduct or the TOFA raw material. In fact, as described in U.S. Pat. No.3,925,342 to Scharrer, incorporated herein by reference, it was observedthat the dimerization by-product from the clay-based dimerization ofTOFA has very limited utility due to the presence of the branched chainisomers produced during the dimerization process, the TOFA beingcharacterized as containing principally straight chain fatty acids.

However, it has now been discovered that the dimerization distillateby-product does in fact provide an excellent alternative to TOFA as wellas the traditional substantially pure stearic or oleic acid materials asa promoter for vulcanization reactions. When it is realized that recentincreased production of TOFA dimer products for other uses willinevitably result in substantially increased accumulation of themonomeric distillate by-product, the advantages of the invention whichprovide a highly beneficial utility for this material become verysignificant. Also, disposal problems associated with increasedproduction of the TOFA dimer by-products are considerably lessened andthe goal of full utilization of components of the renewable resource(wood) from which these products are derived is furthered to a greatextent.

The invention is applicable to a wide range of elastomers including, butnot limited to, polymers, copolymers and terpolymers of substituted andunsubstituted olefinic compounds, arene compounds and olefinic and arenecompounds such as natural and synthetic rubber. Synthetic rubbersinclude polybutadiene rubber, polyisoprene rubber, styrene-butadienerubber, butyl rubber, halogenated butyl rubbers such as chlorobutyl andbromobutyl rubber, chlorinated polyethylene rubber, chlorosulfonatedpolyethylene, nitrile rubber, chloroprene rubber, ethylene-propyleneterpolymer rubber such as ethylene-propylene-diene monomer, siliconerubber, neoprene rubber, polysulfides, polyacrylate rubber,epichlorohydrin rubber, fluoroelastomers, polyurethanes andthermoplastic rubber.

The amount of elastomer present in an elastomeric formulation may varywidely depending on the desired product qualities. Typically, theelastomeric formulation will contain from about 55 to about 85% byweight of elastomer which may be a single elastomer or a mixture ofelastomers.

Elemental sulfur is the most commonly used sulfur source for providingthe vulcanizing agent, however, an organic sulfur compound such as anolefin adduct, an amine disulfide or a polymeric polysulfide may also beused as a source of sulfur. The amount of sulfur required in theelastomeric formulation to produce substantial crosslinking may rangefrom about 0.5 to about 5% by weight free sulfur based on the totalweight of the formulation.

It is known that low sulfur ratios tend to improve the resistance of thevulcanizate to deterioration during aging. Accordingly, when it isdesired to produce elastomeric compositions having a low sulfur ratioand short curing time, organic accelerators are often combined withvulcanization promoters such as stearic acid or oleic acid and/or metaloxide activators such as zinc oxide, calcium oxide, magnesium oxide andlead oxide.

According to a preferred embodiment of the invention, the vulcanizationpromoter is provided by the monomeric distillate by-product from theclay-catalyzed dimerization of TOFA. The distillate by-product from theclay-based dimerization of TOFA is believed to be composed of a mixtureof C₁₆ to C₁₈ monounsaturated fatty acids which are predominantlyrandomly branched isomers having the single double bond located atrandom positions in the molecules. If the distillate by-product isobtained from a fatty acid mixture other than tall oil, the by-productmay contain a significant quantity of branched C₂₀ to C₂₂monounsaturated fatty acids. The by-product will typically be effectiveas a promoter in a concentration ranging from about 0.1 to about 6% byweight based on the total weight of the composition mixture prior tovulcanization. Preferably, the by-product comprises about 0.7 to about1.4 wt. % of the pre-vulcanized mixture.

The clay-catalyzed dimerization of TOFA is conventionally carried out bycontacting TOFA with a naturally occurring bentonite, montmorilloniteclay, classified by Chemical Abstracts Number (CAS#) 1302-78-9 in areaction vessel. The reaction mixture is held at a temperature in therange of from about 200° C. to about 260° C., under steam pressure inthe range of from about 3.4 atm. to about 6.8 atm., for about 4 hours.After the reaction is substantially complete, the clay is filtered outleaving a reaction product comprising unreacted monomeric material(about 30 to 40% of the original TOFA) as a by-product anddimeric/trimeric fatty acid material. The monomeric material isdistilled off at from about 250° C. to about 280° C. under vacuum offrom about 1 millimeter to about 2 millimeters of mercury yielding themonomeric material as a distillate by-product with the dimeric/trimericfatty acids remaining as bottoms.

Without being bound by theory, it is believed that in the acidicenvironment on the surface of the clay in the dimerization process, aportion of the original straight chain fatty acids, mostly fatty acidscontaining from about 16 to about 18 carbon atoms, are rearranged to theiso- or branched-chain acids and that these rearranged acids form thechief constituents of the monomeric distillate by-product. It is nowbelieved that these iso- or branched-chain fatty acids in the by-productexhibit a relatively high degree of monounsaturation and contain arelatively small percentage of di- or poly- unsaturated fatty acids.

Typically, elastomers are compounded prior to vulcanization or curing inorder to provide the most suitable combination of materials in theircorrect proportion so that the finished elastomeric product is of thedesired quality and is produced at the lowest cost possible.Accordingly, the processes of mixing, forming and curing are essentialand well-known fabrication steps for commercially significantelastomers.

The mixing operation is important because processing steps subsequent tomixing depend on an adequate and substantially uniform mixture ofingredients. The primary objectives of mixing include obtaining auniform blend of ingredients, attaining adequate dispersion of fillersand producing consecutive uniform batches having similar degrees ofviscosity and dispersion of the filler. The ingredients may be mixedusing roll mills and internal mixers, e.g., a BANBURY mixer availablefrom Farrel Corporation of Ansonia, Connecticut, an INTERMIX mixeravailable from Francis Shaw & Company, Ltd. of Manchester, England or aBOLLING mixer available from Kobelco Steward Bolling, Inc. of Cleveland,Ohio for batch mixing operations. Continuous mixers such as a FARRELmixer available from Farrel Corporation, a KO-KNEADER mixer availablefrom B&P Process Equipment and Systems, LLC of Saginaw, Mich. or aKNEADERMASTER mixer available from Gorman-Rupp Company of Mansfield,Ohio may also be used.

It is important during compounding or mixing to prevent any appreciablecrosslinking of the elastomer so that the ingredients are homogeneouslymixed without a substantial increase in viscosity of the formulation. Inorder to prevent premature crosslinking, delayed-reaction acceleratorsare used. Thus, compounds which are commonly used as acceleratorsgenerally do not act as accelerators initially, but undergo a chemicalreaction during the rubber processing step to produce the activeaccelerator species. When more delay in crosslinking is required, avulcanization retarder can also be used.

A wide variety of accelerators may be used in rubber compounding,however, the most common is comprised of derivatives of2-mercaptobenzothiazole. Accordingly, the accelerators may be selectedfrom the group consisting of 2-mercaptobenzothiazole,bis(2,2-benzothiazolyl)disulfide,N-tert-butyl-2-benzothiazolesulfenamide,N-cyclohexyl-2-benzothiazolesulfenamide,2-(4-morpho-linylthio)benzothiazole, zinc O,O-di-n-butylphosphorodithioate, 1,3-diphenyl-guanidine, ethylenethiourea(2-imidazolidinethione), tetramethylthiuram disulfide,tetramethyl-thiuram monosulfide, andN-oxydiethylenethiocarbamyl-N′-oxydiethylene-sulfen amide. The amount ofaccelerator in the formulation may range from about 0.4 to about 0.8% byweight of the total formulation.

A suitable retarder for elastomeric matrixes includesN-(cyclohexylthio)phthalimide. Other retarders include salicylic acidand phthalic anhydride. The amount of retarder used in the formulationmay range from about 0.1 to about 0.5% by weight of the totalformulation.

Reinforcing aids or fillers may also be used to improve the propertiesof the vulcanizates. The most common reinforcing aids are carbon blackand silica. The degree of reinforcement increases with a decrease in theparticle size of the filler. Silica and/or carbon black may be used inthe elastomeric formulation in an amount ranging from about 2 to about40 percent by weight of the formulation. The silica may be any silicaconventionally used in elastomeric compositions including precipitatedsilica such as fumed or pyrogenic silica composed of from about 80 toabout 99.9% by weight SiO₂. It is preferred that the silica be in arelatively fine, subdivided state so that it has an average particlesize in the range of from about 1 to about 100 nm for optimal dispersionin the elastomeric material. The silica generally may have a surfacearea ranging from about 45 to about 700 m²/gram, preferably from about100 to about 300 m²/gram, and a bulk density ranging from about 0.03 toabout 0.3 gram/cm³.

In addition to the fillers and accelerators, the elastomericcompositions may include softeners, tackifying agents, extenders,plasticizers, bonding or reinforcing agents, antioxidants andstabilizers. These ingredients are typically used in minor amounts inthe formulation and may comprise only about 10 weight percent or less ofthe total weight of the formulation.

In order to prepare a cured elastomer according to the invention, aformulation comprising an elastomer, a vulcanizing agent, anaccelerator, a vulcanization promoter and, optionally, a filler, isprepared by combining the ingredients in any order in a high shear mixersuch as a BANBURY mixer. Once the ingredients are sufficiently mixed,the elastomeric formulation may be formed, molded, calendered and/orextruded. Subsequent to forming, molding, calendering and/or extrusion,heat is applied to the formulation to vulcanize the elastomer bypromoting cross-link bonds in the elastomer. The formulation may becured by heating to a temperature ranging from about 140° to about 160°C. for a period of time ranging from about 15 minutes to about 30minutes.

The following nonlimiting examples further illustrate various aspects ofthe invention.

EXAMPLES

Rubber formulations were compounded and cured at 149 ° C. for 20 minutesin order to produce specimens according to ASTM D 3182 for use incomparing rheological properties of the cured rubbers. One set ofsamples was made from formulations containing stearic acid while theother set of samples was made from formulations containing the monomericdistillate fraction from a clay-based dimerization process for tall oilfatty acid. The formulations were cross-linked with elemental sulfuravailable from Harwick Chemical Corporation of Akron, Ohio and arecontained in the following table.

TABLE 1 Sample Sample Sample Sample Sample Sample #1 #2 #3 #4 #5 #6Component (wt. %) (wt. %) (wt. %) (wt. %) (wt. %) (wt. %) SIR-10¹ 41.841.8 41.5 41.5 41.8 41.8 PBD 220² 17.9 17.9 17.8 17.8 17.9 17.9 N 660Carbon 29.9 29.9 29.7 29.7 29.9 29.9 Black³ FLEXON 111⁴ 4.2 4.2 4.2 4.24.2 4.2 AGERITE 0.6 0.6 0.6 0.6 0.6 0.6 SUPERLITE⁵ DUREZ 1.8 1.8 1.8 1.81.8 1.8 29095⁶ OBTS⁷ 0.5 0.5 0.5 0.5 0.5 0.5 sulfur⁸ 1.2 1.2 1.2 1.2 1.21.2 KADOX 911⁹ 1.4 1.4 1.4 1.4 0.7 0.7 INDUSTRENE — 0.7 — 1.4 — 1.4 R¹⁰SYLFAT D-1¹¹ 0.7 — 1.4 — 1.4 — ¹SIR-10 is Standard Indonesian naturalrubber, grade 10 available from Akrochem Corporation of Akron, Ohio.²PBD 220 is polybutadiene rubber commercially available from BayerCorporation of Pittsburgh, Pennsylvania. ³N660 Carbon Black iscommercially available from Cabot Corporation of Atlanta, Georgia.⁴FLEXON 111 is a high aromatic petroleum oil used as a plasticizer orextender commercially available from Exxon Chemical Company of Houston,Texas. ⁵AGERITE SUPERLITE is a polyalkyl polyphenol antioxidantcommercially available from R. T. Vanderbilt Company of New York, NewYork. ⁶DUREZ 29095 is an alkyl phenol-formaldehyde thermoplastictackifying agent commercially available from Hooker Chemical & PlasticsDiv. of Tonawanda, New York. ⁷OBTS is an N-oxydiethylene-2-benthiazolesulfenamide accelerator commercially available from Akrochem Corporationof Akron, Ohio. ⁸Sulfur is available form Harwick Chemical Corporationof Akron, Ohio. ⁹KADOX 911 is zinc oxide accelerator activatorcommercially available from New Jersey Zinc Company of Bethlehem,Pennsylvania. ¹⁰INDUSTRENE R is a stearic acid crosslinking agent havingan acid number of 199, an iodine value of 5, 0.6% by weightunsaponifiables and containing 76 wt. % stearic acid and 18 wt. %palmitic acid as determined by GLC analysis commercially available fromHumko of Memphis, Tennessee. ¹¹SYLFAT D-1 is a monomeric distillateproduct from the clay-based dimerization of TOFA having an acid value of170 to 185, an iodine value of about 73, 6.8 wt. % unsaponifiables, 1-2wt. % palmitic acid, 34-41 wt. % isostearic/isooleic acid, 12 to 17 wt.% stearic acid, 8 to 11 wt. % oleic acid and 20-26 wt. % other acids andis commercially available from Arizona Chemical Company of Panama City,Florida.

The above rubber formulations were tested for their rheologicalproperties according to ASTM D 2084-93 using a TECH PRO MP-100 rheometeravailable from Diversified Data Corporation of Springfield, Va. Therheological properties of the foregoing cured samples are shown in Table2.

TABLE 2 Rheometer Sample Sample Sample Sample Sample Sample Data #1 #2#3 #4 #5 #6 max. torque 25.75 27.10 27.04 28.44 26.40 27.68 (MH, Lbfinch) min. torque 6.20 6.32 5.56 6.03 5.85 6.03 (ML, Lbf inch) Scorchtime 8.25 8.33 8.50 8.83 8.58 8.46 (TS 2, minutes) Cure time 12.21 12.3312.54 12.92 12.17 12.42 (TC 50%, minutes) Cure time 16.50 17.04 17.7918.67 17.25 17.96 (TC 90%, minutes)

The foregoing example demonstrates that a rubber formulation cured witha monomeric distillate fraction obtained from the clay-baseddimerization of TOFA (Samples 1, 3 and 5) is surprisingly comparable orhas even slightly better rheological properties than the samplescontaining stearic acid as a vulcanization promoter (Samples 2, 4 and6). Cure times for samples 1, 3 and 5 were also slightly better than thecure times for samples 2, 4 and 6.

Physical properties of cured test plaques were determined for thesamples. The tear strength of the samples was determined using ASTM DieB specimens at a 20 inch per minute crosshead speed. The other tensilestrength properties were determined using ASTM Die C dumbbell specimensat the same crosshead speed. The results of the physical tests are givenin the following table.

TABLE 3 Tensile Sample Sample Sample Sample Sample Sample Properties #1#2 #3 #4 #5 #6 tear strength, 497 471 583 587 538 534 ppi ultimate 670650 650 610 620 630 elongation (%) 100% modulus, 220 240 230 270 230 260psi 200% modulus, 550 600 580 660 570 650 psi 300% modulus, 1050 11301110 1230 1090 1190 psi 400% modulus, 1630 1740 1730 1910 1700 1810 psi500% modulus, 2290 2390 2430 2610 2350 2480 psi tensile strength, 34503380 3480 3340 3260 3410 psi Shore A 54 56 53 58 53 56 durometer points

As shown by the physical properties in the foregoing table, samplesprepared using the monomeric distillate by-product also had slightlyhigher tensile strength, slightly lower modulus and slightly lowerdurometer than the samples prepare using stearic acid. However, thedifferences do not appear to be statistically significant and for allpractical purposes, may be considered the same. Accordingly, themonomeric distillate by-product, which is less costly than TOFA, stearicand oleic acids, has no manifest disadvantages in terms of compounding,compatibility or stability with respect to use of the by-product as avulcanization promoter.

Although various embodiments and features of the invention have beendescribed in the foregoing detailed description, those of ordinary skillwill recognize the invention may be capable of numerous modifications,rearrangements and substitutions without departing from the scope of theinvention as set forth in the appended claims.

What is claimed is:
 1. An elastomeric composition comprising avulcanizable elastomer, sulfur and from about 0.1 to about 6% by weightof a monomeric distillate by-product from the clay-catalyzeddimerization of fatty acids wherein the by-product includes asubstantial portion of unsaturated branched-chain isomers.
 2. Thecomposition of claim 1 wherein the by-product is obtained fromclay-catalyzed dimerization of tall oil fatty acids.
 3. The compositionof claim 2 wherein the monomeric by-product contains from about 30 toabout 45% by weight of unsaturated branched-chain isomers.
 4. Thecomposition of claim 2 wherein the branched-chain isomers have fromabout 16 to about 18 carbon atoms.
 5. The composition of claim 1 whereinthe by-product contains a substantial portion of unsaturatedbranched-chain isomers having from about 16 to about 18 carbon atoms. 6.The composition of claim 1 wherein the by-product contains a substantialportion of unsaturated branched-chain isomers having from about 20 toabout 22 carbon atoms.
 7. The composition of claim 1 further comprisinga carbon black filler.
 8. The composition of claim 1 further comprisinga silica filler.
 9. The composition of claim 1 wherein the elastomer isselected from the group consisting of isoprene, butadiene,methylbutadiene, dimethylbutadiene, pentadiene, polybutadiene,polyisoprene and copolymers of the foregoing with styrene, methylstyrene, olefins selected from the groups consisting of ethylene,propylene and isobutylene and vinyl compounds selected from the groupconsisting of vinylchloride, acrylic acid, acrylonitrile,methacrylonitrile and methacrylic acid.
 10. The composition of claim 1further comprising a filler comprised of a mixture of carbon black andsilica.
 11. A cross-linked elastomeric product made by a process whichcomprises compounding a mixture including a vulcanizable elastomer,sulfur and from about 0.1 to about 6% by weight of a monomericdistillate by-product from the clay-catalyzed dimerization of fattyacids, the by-product including a substantial portion of unsaturatedbranched-chain isomers, and heating the compounded mixture to atemperature sufficient to cause substantial cross-linking of theelastomer.
 12. The elastomeric product of claim 4 wherein the fattyacids are tall oil fatty acids.
 13. The elastomeric product of claim 11wherein the elastomer is selected from the group consisting of isoprene,butadiene, methylbutadiene, dimethylbutadiene, pentadiene,polybutadiene, polyisoprene and copolymers of the foregoing withstyrene, methyl styrene, olefins selected from the groups consisting ofethylene, propylene and isobutylene and vinyl compounds selected fromthe group consisting of vinylchloride, acrylic acid, acrylonitrile,methacrylonitrile and methacrylic acid.
 14. The elastomeric product ofclaim 11 wherein the by-product contains from about 30 to about 45% byweight of unsaturated branched-chain isomers having from about 16 toabout 18 carbon atoms.
 15. The elastomeric product of claim 11 whereinthe mixture further comprises carbon black, silica or a mixture ofcarbon black and silica.
 16. A cross-linked elastomeric product whichcomprises a cross-linked elastomer, sulfur and from about 0.1 to about6% by weight of a monomeric distillate by-product from theclay-catalyzed dimerization of fatty acids wherein the by-productincludes a substantial portion of unsaturated branched-chain isomers.17. The elastomeric product of claim 16 wherein the fatty acids are talloil fatty acids.
 18. The elastomeric product of claim 16 wherein theelastomer is selected from the group consisting of isoprene, butadiene,methylbutadiene, dimethylbutadiene, pentadiene, polybutadiene,polyisoprene and copolymers of the foregoing with styrene, methylstyrene, olefins selected from the groups consisting of ethylene,propylene and isobutylene and vinyl compounds selected from the groupconsisting of vinylchloride, acrylic acid, acrylonitrile,methacrylonitrile and methacrylic acid.
 19. The elastomeric product ofclaim 16 wherein the monomeric distillate contains from about 30 toabout 45% by weight of unsaturated branched-chain isomers having fromabout 16 to about 18 carbon atoms.
 20. The elastomeric product of claim16 further comprising carbon black, silica or a mixture of carbon blackand silica.
 21. An elastomeric composition comprising a vulcanizableelastomer, sulfur and from about 0.1 to about 6% by weight of amonomeric distillate by-product from the clay-catalyzed dimerization offatty acids wherein the by-product is obtained from the clay-catalyzeddimerization of fatty acids of oils selected from the group consistingof linseed, rapeseed, canola, soybean, tall oils, and mixtures of two ormore such oils.
 22. A cross-linked elastomeric product made by a processwhich comprises compounding a mixture including a vulcanizableelastomer, sulfur and from about 0.1 to about 6% by weight of amonomeric distillate by-product from the clay-catalyzed dimerization offatty acids and heating the compounded mixture to a temperaturesufficient to cause substantial cross-linking of the elastomer whereinthe fatty acids are selected from the group consisting of linseed,rapeseed, canola, soybean, tall oils, and mixtures of two or more suchoils.
 23. A cross-linked elastomeric product which comprises across-linked elastomer, sulfur and from about 0.1 to about 6% by weightof a monomeric distillate by-product from the clay-catalyzeddimerization of fatty acids wherein the fatty acids are obtained fromthe group consisting of linseed, rapeseed, canola, soybean and talloils.